Not applicable.
This invention is in the field of microstructure manufacture, and is more specifically directed to the manufacture of rotatable microstructures, such as micromirror assemblies for optical wireless communications.
In recent years, techniques have been developed for the fabrication of movable microstructures, resulting in the capability of micromachines. These techniques generally involve the selective etching of a monolithic body, for example a silicon wafer. The manufacturing processes are similar to those used in the fabrication of integrated circuits, including photolithography to define the locations at which the wafers are to be etched to define the microstructure. The silicon material may be etched to produce features that are sufficiently fine to permit repeated flexure, and thus serve as hinges. Positioning of the hinged microstructure may be magnetically controlled.
One application of such positionable microstructures is in an optical wireless communications network. According to this approach, data is transmitted by way of modulation of a light beam, in much the same manner as in the case of fiber optic telephone communications. A photoreceiver receives the modulated light, and demodulates the signal to retrieve the data. The aiming of the light beam may be carried out by way of a positionable micromirror such as described in copending application Ser. No. 09/310,284, filed May 12, 1999, entitled xe2x80x9cOptical Switching Apparatusxe2x80x9d, commonly assigned herewith and incorporated herein by this reference. As disclosed in this application, the micromirror reflects the light beam in a manner that may be precisely controlled by electrical signals. As disclosed in this patent application, the micromirror assembly includes a silicon mirror capable of rotating in two axes. One or more small magnets are attached to the micromirror itself; a set of four coil drivers are arranged in quadrants, and are current-controlled to attract or repel the micromirror magnets as desired, to tilt the micromirror in the desired direction. These single micromirror assemblies in the optical transmitter modules provide good communications in many applications.
However, practical and regulatory limits on the power density of the transmitted beam in turn limit the signal energy that may be communicated using these single beam steering elements. For example, an important power density limit is that defining the xe2x80x9ceye-safexe2x80x9d power density of the transmitted beams; use of a power density above this limit requires significant facility modifications (e.g., warning lights, eye protection, etc.), which are inconsistent with use of the system for data communications in office and building-to-building environments. Reduction of the power density of the transmitted beam by increasing the beam cross-sectional diameter requires a corresponding increase in the size of the micromirror beam steering element. As known in the telescope art, however, the construction of accurate mirrors with larger diameters is an increasingly difficult task. As such, according to conventional technology, it becomes very costly to increase the signal power of an optical communications beam while maintaining the power density below safety and other limits.
In the manufacture of micromirror assemblies, as in the manufacture of any microstructures, mechanical damage of the microstructure is a primary cause of yield loss, and thus directly affects the manufacturing cost of the microstructures. In the case of rotatable micromirrors, for example, mechanical damage especially occurs during attachment of the permanent magnets. In particular, it has been observed that the integrated torsional hinges are frequently damaged during the placement of the individual micromirror elements into chip trays or other fixtures in which the permanent magnets are attached according to conventional methods. Indeed, it has been observed that a drop of a micromirror from a height of only 1 to 2 mm is enough to break a hinge. Equivalent manufacturing operations for other classes of microstructures, where actuating devices are attached to the structures, are also prone to damage the fragile microstructures and cause yield loss.
It is an object of the present invention to provide a method of manufacturing microstructures in such a way that the structures are less vulnerable to mechanical damage.
It is a further object of the present invention to provide a method of mounting permanent control magnets to an array of micromirrors while yet in wafer form.
It is a further object of the present invention to provide such a method of mounting with improved manufacturing yield.
It is therefore an object of the present invention to provide such a method that may be applied to the fabrication of a mirror assembly that can accurately steer a relatively large optical beam in the communication of optical data.
These and other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.
According to the present invention, an array of rotatable microstructures may be assembled and mounted in combination with its permanent magnets, in wafer form. A carrier wafer is first prepared, with holes etched therethrough to receive the eventual bottomside permanent magnets. The wafer from which the structures are to be formed is then attached to the carrier wafer, and the structures are formed into this wafer. For the example of micromirrors, the mirrors, hinges, and gimbals are formed into this wafer. Magnets are then attached to the bottomside of the structures at the location of the etched holes in the carrier wafer; opposing topside magnets may also be attached to the structure wafer. The structure wafer may then be released from the carrier wafer, to yield the array of microstructures with attached magnets.